Project Summary Congenital heart disease occurs in approximately 32,000 infants per year. The American Heart Association estimates that 300,000 children under age 21 have a congenital heart defect, and approximately thirty-eight percent of these will have one or more surgical procedures. This cohort of infants undergoing cardiac surgery represents an orphan disease population by NIH and FDA definitions. Over the past 20 years increasing numbers of patients have undergone cardiopulmonary bypass for correction of congenital heart defects and or palliation of single ventricle type anatomy. Technical advances have been made in these surgical procedures. However, progress has been limited with regard to pharmacological management of infants during the postoperative period. The recent Triiodothyronine Supplementation in Infants and Children Undergoing Cardiopulmonary Bypass (TRICC) clinical trial demonstrated that triiodothyronine supplementation with Triostat (liothyronine) dramatically reduced time on the ventilator after surgical procedures using cardiopulmonary bypass infants under 5 months of age. In this Phase III trial we will obtain confirming evidence by performing a randomized placebo controlled trial examining triiodothyronine supplementation with Triostat (liothyronine) in infants under 5 months of age.